The fluorite to pyrochlore phase transition in La2Zr2O7 has been studied in the literature for decades in the context of thermal barrier coatings and reinforcement materials. However, the nature of the phase transition in this system is still not well understood. In this article we have investigated the phase transition in La2Zr2O7, calcined at different temperatures, using powder x-ray diffraction and Raman measurements. Rietveld analyses of the x-ray data suggest a fluorite to pyrochlore phase transition in the system occurring between 1000-1450oC. Nonetheless, Raman spectra, recorded with different excitation wavelengths (λ_ex) reveal that the dynamics of phase transition is different in the near-surface region and in the bulk inside. While the near-surface region carried the signatures of a pure pyrochlore phase, the temperature dependent Raman measurements with λ_ex=785 nm suggest that locally formed oxygen related disordered states are present in the bulk of the system.
Improved thermomechanical properties have been reported for various high-entropy oxides containing typically five metal cations. This study further investigates a series of duodenary (11 metals + oxygen) high-entropy oxides by mixing different fractions of a five-cation fluorite-structured niobate and a seven-cation pyrochlore (both containing Yb) with matching lattice constants. Nine compositions of duodenary high-entropy oxides have been examined. All of them exhibit single high-entropy phases of either disordered fluorite or ordered pyrochlore structure. An order-disorder transition (ODT) i…
DFT study of La$_2$Ce$_2$O$_7$: disordered fluorite vs pyrochlore structure
用\textit{ab initio}密度泛函理论（DFT）计算了镧铈氧化物（La $_2$ Ce $_2$ O $_7$）的晶体结构.通过比较它们的形成能，研究了类萤石和类辉石岩结构的相对稳定性。这些形成能表明烧绿石结构优于萤石结构，显然与基于实验中子和X射线衍射（XRD）的结论相矛盾。通过计算和比较一组不同有序和随机结构的XRD谱，我们表明烧绿石结构与衍射实验是一致的。基于这些原因，我们建议烧绿石结构作为La $_2$ Ce $_2$ O $_7$的基态晶体结构。%，我们表明，在这项工作中考虑的结构，烧绿石的几何形状显然是有利的无序萤石的几何形状。
This invited talk will review a series of our recent studies on high-entropy ceramics (HECs) and compositionally complex ceramics (CCCs) and discuss the future perspective. Various single-phase equimolar quinary (five-component) HECs, e.g., MB2, MB, M3B4, MB4, and MB6 borides, MSi2 and M3Si5 silicides, perovskite, fluorite, and pyrochlore oxides, and (intermetallic) aluminides have been fabricated. We further proposed to extend HECs to CCCs to include non-equimolar compositions and further consider short- and long-range orders, which reduce configurational entropies but offer additional dimens…
We present an aberration corrected scanning transmission electron microscopy (ac-STEM) analysis of the perovskite (LaFeO3) and pyrochlore (Yb2Ti2O7 and Pr2Zr2O7) oxides and demonstrate that both the shape and contrast of visible atomic columns in annular dark-field (ADF) images are sensitive to the presence of nearby atoms of low atomic number (e.g. oxygen). We show that point defects (e.g. oxygen vacancies), which are invisible - or difficult to observe due to limited sensitivity - in X-ray and neutron diffraction measurements, are the origin of the complex magnetic ground state of pyrochlore…
This study discovers a reversible order-disorder transition (ODT) in a 10-cation compositionally complex ceramic, (Nd0.15Pr0.15Dy0.8Ho0.8Er0.8Ti0.2Yb0.1Hf0.1Zr0.1Nb0.8)O7-delta, induced via annealing in oxidized vs. reduced environments at 1600C. Notably, the 10-cation oxide remains a homogenous single-phase high-entropy solid solution before and after the ODT in the pyrochlore vs. fluorite structure that can be quenched. In-situ neutron diffraction reveals the oxygen vacancy formation and atomic displacement during this ODT. The temperature dependence of thermal conductivity is altered by the…
The pyrochlore oxides Dy$_{2}$Ti$_{2}$O$_{7}$ and Ho$_{2}$Ti$_{2}$O$_{7}$ are well studied spin ice systems and have shown the evidences of magnetic monopole excitations. Unlike these, Dy$_{2}$Zr$_{2}$O$_{7}$ is reported to crystallize in a distorted fluorite structure. We present here the magnetic and heat capacity studies of La substituted Dy$_{2}$Zr$_{2}$O$_{7}$. Our findings suggest the absence of spin ice state in Dy$_{2}$Zr$_{2}$O$_{7}$ but the emergence of the magnetic field induced spin freezing near T $\approx$ 10 K in ac susceptibility measurements which is similar to Dy$_{2}$Ti$_{2}…
Certain pyrochlore oxides are among the best oxygen catalysts in alkaline media. Hence, exploring epitaxial films of these materials is of great fundamental and technological interest. Unfortunately, direct film growth of one of the most promising pyrochlores, Bi$_2$Pt$_2$O$_7$, has not yet been achieved, owing to the difficulty of oxidizing platinum metal in the precursor material to Pt$^{4+}$. In this work, in order to induce oxidation of the platinum, we annealed pulsed laser deposited films consisting of epitaxial $δ$-Bi$_2$O$_3$ and co-deposited, comparatively disordered platinum. We pres…
We report the decomposition of La2-xSrxCuO4 into La2O3 and Cu nanoparticles in ultra-high vacuum, observed by in-situ heating experiments in a transmission electron microscope (TEM). The analysis of electron diffraction data reveals that the phase decomposition process starts at about 150 °C and is considerably expedited in the temperature range of 350-450 °C. Two major resultant solid phases are identified as metallic Cu and La2O3 by electron diffraction, simulation, and electron energy-loss spectroscopy (EELS) analyses. With the aid of calculations, La2O3 phases are further identified to be …
While pyrochlore iridate thin films are theoretically predicted to possess a variety of emergent topological properties, experimental verification of these predictions can be obstructed by the challenge in thin film growth. Here we report on the pulsed laser deposition and characterization of thin films of a representative pyrochlore compound Bi2Ir2O7. The films were epitaxially grown on yttria-stabilized zirconia substrates and have lattice constants that are a few percent larger than that of the bulk single crystals. The film composition shows a strong dependence on the oxygen partial pressu…
The discovery of new topological electronic materials brings a chance to uncover novel physics and plays a key role in observing and controlling various intriguing phenomena. Up to now, many materials have been theoretically proposed and experimentally proved to host different kinds of topological states. Unfortunately, there is little convincing experimental evidence for the existence of topological oxides. The reason is that oxidation of oxygen leads to ionic crystal in general and makes band inversion unlikely. In addition, the realization of different topological states in a single materia…
Developing good performance and low-cost oxygen permeable membranes for CO2 capture based on the oxy-fuel concept is greatly desirable but challenging. Despite tremendous efforts in exploring new CO2-stable dual-phase membranes, its presence is however still far from meeting the industrial requirements. Here we report a series of new Ca-containing CO2-resistant oxygen transporting membranes with composition 60wt.%Ce0.9Ln0.1O2-40wt.%Ln0.6Ca0.4FeO3(CLnO-LnCFO; Ln = La, Pr, Nd, Sm) synthesized via a Pechini one-pot method. Our results indicate all investigated compounds are composed of perovskite…
Pyrochlore erbium titanate samples in the form of powders (with nominal compositions of Er2Ti2+xO7) and bulk single crystals were prepared to elucidate the effect of synthesis and growth conditions on their composition, structure, and transition temperature. All samples were characterized using X-ray diffraction and specific heat measurements. Larger lattice parameters were measured for the Ti-deficient stuffed powders, Er2(Ti2-xErx)O7-δ, while Ti-rich anti-stuffed powders, (Er2-xTix)Ti2O7+δ, showed a decrease in lattice parameter. The lattice parameter for the stoichiometrically synthesized p…
We used classical and accelerated molecular dynamics simulations to characterize vacancy-mediated diffusion of cations in Gd$_2$Ti$_2$O$_7$ pyrochlore as a function of the disorder on the microsecond timescale. We find that cation vacancy diffusion is slow in materials with low levels of disorder. However, higher levels of disorder allow for fast cation diffusion, which is then also accompanied by fast antisite annihilation and ordering of the cations. The cation diffusivity is therefore not constant, but decreases as the material reorders. The results suggest that fast cation diffusion is tri…
We have investigated the effect of synthesis and growth conditions on the magnetic, structural, and compositional properties of pyrochlore oxide holmium titanate and demonstrate a method for growing high quality stoichiometric single crystals. A series of polycrystalline samples with various contents of Ti (-0.08 \leqslant x \leqslant 0.08, and nominal compositions of Ho2Ti2+xO7) were synthesized at different temperatures, and characterized using powder X-ray diffraction. The results show that synthesizing powders at a higher temperature of 1500 °C yield single phase compounds. Ti deficient po…
In this work we present a new chemical route to synthesize Tm2Mn2O7 pyrochlore, a compound which is thermodynamically unstable at ambient pressure. Differently from the reported in the past high pressure synthesis of the same compound applying oxides as starting materials, we have obtained a pure phase Tm2Mn2O7 by a converting TmMnO3 at 1100 C and 1300 bar oxygen pressure. The studies of Tm2Mn2O7 performed by a high resolution neutron powder diffraction have shown, that a pure pyrochlore cubic phase Tm2Mn2O7 (space group Fd-3m) have been obtained. On cooling below 25K there is a transition to …
Tuning electronic properties through strain engineering of metal oxides is an important step toward understanding electrochemical and catalytic reactions in energy storage and conversion devices. Traditionally, strain engineering studies focused on movement of oxygen ions at high temperatures (500 °C and above), complicating electrical properties by introducing mixed electronic and ionic conductivity. In this study, we demonstrate room temperature charge redistribution in a CeO2 thin film as a result of phase transformation in a localized region by mechanical deformation. Mechanical indentatio…
Low temperature Spin freezing and Diffuse Magnetic Correlations in Tb$_{2}$Zr$_{2-x}$Ti$_{x}$O$_{7}$ (x = 0, 0.5)
Indian Institute of Technology Mandi（印度理工学院曼迪分校）；Heidelberg University（海德堡大学）；ISIS Neutron and Muon Source, STFC Rutherford Appleton Laboratory（STFC拉瑟福德阿普尔顿实验室ISIS中子与缪子源）；Center for Quantum Science and Technologies, Indian Institute of Technology Mandi（印度理工学院曼迪分校量子科学与技术中心）
X-ray absorption spectroscopy (XAS) as well as x-ray magnetic circular dichroism (XMCD) and magnetization of hybrid pyrochlore Eu2-xFexTi2O7 were investigated, where the rare earth Eu (4f) was replaced with transition metal Fe (3d) to introduce competing 4f-3d interactions. It is confirmed that the valence states of Eu and Fe ions are formally trivalent while that of Ti ions are tetravalent (3d0). The analysis yielded that the tetravalent Ti ions occupy octahedral sites with distorted Oh symmetry which is triggered by the presence of vacant 8a anionic site adjacent to TiO6 octahedra. Further s…
